1. Field of the Invention
This invention relates to a flexible puncture-resistant heat shrinkable multilayer packaging film having particular utility for forming primal meat bags which are highly puncture-resistant in character.
2. Description of the Prior Art
In the art of packaging primal red meat cuts for shipment and/or storage prior to final butchering, it has been common practice to use heat-shrinkable multilayer film bags for packaging of the primal cuts. The multilayer films commonly employed for such purpose typically include a layer serving as an oxygen barrier to inhibit spoilage and discoloration of the meat by oxidation, with other layers in the film being employed to provide adequate strength, abrasion resistance, and good low temperature characteristics for the composite film material. The properties desirably possessed by such film materials include high puncture resistance, satisfactory heat-shrink characteristics, and high resistance to delamination at elevated temperatures and during the heat shrinking operation.
Canadian Pat. No. 982,923, issued Feb. 3, 1966 to Stanley Lustig et al. discloses a heat-shrinkable three-layer film having utility for forming bags suitable for use in shrink packaging primal meat cuts. The disclosed film comprises a first outer layer which consists essentially of a first ethylene/vinyl acetate copolymer having a melt index of from 0.1 to 1.0 decigrams per minute, and a vinyl acetate content of from 10 to 14 weight percent, based on the weight of the first ethylene/vinyl acetate copolymer. The core layer consists essentially of a vinylidene chloride copolymer having at least 65 weight percent of polymerized vinylidene chloride and containing a maximum of 5 weight percent plasticizer, the percentage being based on the weight of the vinylidene chloride polymer. A second outer layer of the film comprises a second ethylene/vinyl acetate copolymer having a melt index of from 0.1 to 1.0 decigrams per minute and a vinyl acetate content of from 12 to 18 weight percent, the percentage being based on the second ethylene/vinyl acetate copolymer.
The aforementioned film may suitably be produced by coextruding the three constituent layers through a tubular die, and then cooling and subsequently biaxially orienting the extrudate to obtain a heat shrinkable film suitable for forming the primal red meat bag. The extrusion and orientation process described in U.S. Pat. No. 3,555,604 issued January, 1971 in the name of H. E. Pahlke may suitably be employed to produce such heat shrinkable film.
The oriented three-layer film, as extruded in tubular form, is formed into bags by flattening the extruded tube, cutting the flattened tubes into desired lengths and heat sealing the sealable layers at one end while leaving the other end open for insertion of primal cuts of fresh red meat. The heat-sealable layers will then form the inner surface of the bag. After the meat is placed in the bag by the meat packer or other processor, the bag is evacuated and the open end thereof sealed in a conventional manner by clipping. The bag is then heat shrunk by exposure to elevated temperature, as for example by passage of the bag through a hot water spray, at a temperature which may suitably be on the order of 90.degree.-95.degree. C.
The primal cut red meat bag described above has demonstrated utility in commercial practice, but suffers from the deficiency that primal cuts of meat containing cut bones sometimes puncture the bag at bone corners or edges. This puncturing occurs even though such bone surfaces are covered with one or more layers of heavy wax impregnated cloth or paper (bone wrap) prior to packaging of the primal cut in the meat bag. Such puncturing of the meat bag by sharp bone surfaces permits the penetration of air from the ambient environment into the previously evacuated bag. This penetration of air causes localized meat spoilage in the vicinity of the bag puncture, thereby requiring additional labor for trimming of the meat prior to its final sale, as well as entailing significant meat losses.
The aforementioned puncturing of the meat bag is due primarily to puncture impact of the bag at bone corners during handling and shipping. Such impact is in turn the result of the bags being dropped or rubbed against hard surfaces such as other meat-containing bags or the interior surfaces of the boxes which are employed for shipment of the packaged meat.
In view of the foregoing, there is a need for packaging films with improved puncture toughness to resist bone puncture. In an attempt to remedy the bone puncture problem, it has been proposed to incorporate polyurethane elastomers in the multilayer film utilized in the primal cut red meat bags, in view of the high impact strength of such elastomeric materials. Generally, such films provide very high impact strength. However, the superior performance of certain grades of thermoplastic polyurethane has been shown to gradually deteriorate with time until most of the initial impact improvement is lost. It is theorized that this phenomena is related to stress-induced crystallinity which adversely affects impact strength. Selection of the polyurethane elastomer is very critical. Polyurethanes which have high impact values as single layer films may not provide the expected improvement in a coextruded film structure. Other polyurethanes which are not suitable by themselves can be improved by the addition of a specially selected elastomer.
Accordingly, it is an object of this invention to provide a multilayer film containing a polyurethane elastomer blend layer, characterized by high impact strength which is retained at desired levels despite the passage of time.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.